A conventional digital signal receiver for receiving a terrestrial wave broadcast transmitted with a digital modulation developed in the United States will be explained. FIG. 13 is a block diagram of this digital signal receiver.
In FIG. 13, a signal received by an antenna is fed into input terminal 101, and is controlled of its amplitude in variable gain amplifier 102. The output of amplifier 102 is mixed with a signal from local oscillator 110 in frequency converter 103, and is converted in frequency to a specified first intermediate frequency (for example, 1407.5 MHz). The output of frequency converter 103 is mixed with a signal from local oscillator 111 in frequency converter 105, and is converted in frequency to a second intermediate frequency (for example, 44 MHz), and is controlled of its amplitude by variable gain amplifier 104. The output of amplifier 104 is converted into a digital signal in analog-to-digital (A/D) converter 106.
Demodulator 107 demodulates the output of A/D converter 106. Equalizer 108 removes distortion due to ghost generated in a transmission line from the output of demodulator 107 and issues it from terminal 109. Level comparator 112 compares the output level of converter 106 with a reference value. The output of converter 106 is filtered by loop filter 113, and is fed into control voltage generator 114. Generator 114 controls the gains of variable gain amplifiers 102 and 104 so that the input level of converter 106 may be constant.
In this block diagram, before synchronism is achieved prior to demodulation, the filter bandwidth of loop filter 113 is broad, and the receiver responds to signal fluctuations at high speed. After the synchronism is established, by narrowing the filter bandwidth, i.e., narrowing a noise bandwidth, the noise characteristic after the synchronization is improved. A similar technology is disclosed, for example, in Japanese Laid-open Patent No. 6-216955.
The control voltage—gain conversion characteristic of the variable gain amplifier is not constant in relation to a control voltage. Therefore, such conventional receiver cannot follow to the level of an input signal fluctuation at high speed in a specific electric field intensity. For example, if an object passes in front of the antenna, and the level of the received signal changes suddenly, the response of the variable gain amplifier cannot follow to it, and a bit error rate degrades.